Image display methods and systems with sub-frame intensity compensation

ABSTRACT

Image display methods and systems are provided wherein only portions of a still image of a motion picture, generated for display during an image time interval, are displayed during respective sub-intervals of the image time interval. A still image is thereby separated into portions and displayed separately during corresponding sub-intervals. Preferably, no single portion of the image includes all of the content of the entire image. However, the sum of all of the portions preferably includes all of the content of the image, and all of the portions are displayed within the image time interval. Although an image displayed in this manner can be correctly perceived by a viewer, recording of such an image may capture fewer than all of the portions of the image to thereby degrade the quality of recorded copies of the image and thus the motion picture. Intensity compensation is performed such that a consistent brightness is perceived by a user notwithstanding the fact that some portions are displayed for less time.

RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.11/079,222 filed Mar. 15, 2005 and claims the benefit of prior U.S.Provisional Application No. 60/552,732 filed Mar. 15, 2004, herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates generally to image display and, in particular, toanti-piracy image display methods and systems.

BACKGROUND

Movie producers are continually becoming more concerned about moviepiracy. In the past year, for example, more than 50 major movies wereillegally copied and publicly released even before they came out intheaters, according to the Motion Picture Association of America(M.P.A.A.). The film industry lost a reported US$3.5 billion last year,and estimated losses may rise to US$5.4 billion this year. Illegallycopied movies filmed during projection, with video cameras or camcordersand similar devices, are a significant contributing factor to revenueloss. While it may not be possible to completely eliminate theft bycopying, it can be advantageous to modify image display and projectiontechniques so that pirated copies are of such low quality that they areof little or no commercial value.

It is known to provide a distinct symbol or watermark on an originalstill image as a means of image or copy identification, so as to enableauthentication of a copy. As examples, U.S. Pat. No. 5,875,249 (Mintzeret al.), U.S. Pat. No. 6,031,914 (Tewfik et al.), U.S. Pat. No.5,912,972 (Barton), and U.S. Pat. No. 5,949,885 (Leighton) disclosemethods of applying a perceptually invisible watermark to image data asverification of authorship or ownership or as evidence that an image hasnot been altered. However, while such methods identify and validateimage data, they provide no direct means of protection against copyingan image, such as using a conventional scanner and color printer.

In contrast, U.S. Pat. No. 5,530,759 (Braudaway et al.) disclosesproviding a visible, color correct watermark that is generated byaltering brightness characteristics but not chromaticity of specificpixels in an image. This approach could be objectionable if used for amotion picture, since the continuing display of a watermark on filmcould annoy an audience and adversely affect the viewing experience.

The above examples for still-frame images illustrate a key problem: aninvisible watermark identifies but does not adversely affect the qualityof an illegal copy, while a visible watermark can be distracting andannoying. With video and motion picture images, there can be yet otherproblems with conventional image watermarking. For example, U.S. Pat.No. 5,960,081 (Vynne et al.) discloses applying a hidden watermark toMPEG data using motion vector data. But this method identifies andauthenticates the original compressed data stream and would not provideidentification for a motion picture that was copied using a camcorder.

Other patents, such as U.S. Pat. No. 5,809,139 (Girod et al.), U.S. Pat.No. 6,069,914 (Cox), and U.S. Pat. No. 6,037,984 (Isnardi et al.)disclose adding an imperceptible watermark directly to the discretecosine transform (DCT) coefficients of a MPEG-compressed video signal.If such watermarked images are subsequently recompressed using a lossycompression method (such as by a camcorder, for example) or are modifiedby some other image processing operation, the watermark may no longer bedetectable.

These particular invisible watermarking schemes add a watermark directlyto the compressed bit stream of an image or image sequence.Alternatively, there are other watermarking schemes that add thewatermark to the image data itself, rather than to the compressed datarepresentation. An example of such a scheme is given in U.S. Pat. No.6,044,156 (Honsinger et al.), which discloses a spread spectrumtechnique using a random phase carrier.

However, regardless of the specific method that is used to embed awatermark, there is always a concern that a watermarking method mightnot be robust, that is, able to withstand various “attacks” that canremove or alter the watermark. Some attacks may be deliberately aimed atthe underlying structure of a given watermarking scheme and requiredetailed knowledge of watermarking techniques applied, although mostattack methods are less sophisticated, performing common modificationsto the image such as using lossy compression, introducing lowpassfiltering, or cropping the image, for example. Such modifications can bemade when a video camera is used to capture a displayed motion picture.These methods present a constant threat that a watermark may be removedduring the recording process.

The watermarking schemes noted above are directed to copyidentification, ownership, or authentication. However, even if awatermarking approach is robust, provides copy control management, andsucceeds in identifying the source of a motion picture, an invisiblewatermark may not be a sufficient deterrent for illegal copying.

As an alternative to watermarking, some copy deterrent schemes used inarts other than video or movie display operate by modifying a signal orinserting a different signal to degrade the quality of any illegalcopies. The modified or inserted signal does not affect playback of alegally obtained manufactured copy, but adversely impacts the quality ofan illegally produced copy. As one example, U.S. Pat. No. 5,883,959(Kori) discloses deliberate modification of a burst signal to foilcopying of a video. Similarly, U.S. Pat. No. 6,041,158 (Sato) and U.S.Pat. No. 5,663,927 (Ryan) disclose modification of expected videosignals in order to degrade the quality of an illegal copy. As yetanother example of this principle, U.S. Pat. No. 4,644,422 (Bedini)discloses adding a degrading signal to discourage copying of audiorecordings. An audio signal having a frequency at and above the highthreshold frequency range for human hearing is selectively inserted intoa recording. The inserted signal is not detectable to the listener.However, any unauthorized attempt to copy the recording onto tapeobtains a degraded copy, since the inserted audio signal interactsadversely with the bias oscillator frequency of a tape recording head.

The above-mentioned copy protection schemes deliberately inject a signalin order to degrade the quality of a copy. While such methods may beeffective for copy protection of data from a tape or optical storagemedium, these methods do not discourage copying of a motion pictureimage using a video camera.

As a variation of the general method where a signal is inserted thatdoes not impact viewability but degrades copy quality, U.S. Pat. No.6,018,374 (Wrobleski) discloses the use of a second projector in videoand motion picture presentation. This second projector is used toproject an infrared (IR) message onto the display screen, where theinfrared message can contain, for example, a date/time stamp, theateridentifying text, or other information. The infrared message is notvisible to the human eye. However, because a video camera has broaderspectral sensitivity that includes the IR range, the message will beclearly visible in any video camera copy made from the display screen.The same technique can be used to distort a recorded image with an“overlaid” infrared image. While the method disclosed in U.S. Pat. No.6,018,374 can be effective for frustrating casual camcorder recording,the method has some drawbacks. A more sophisticated video cameraoperator could minimize the effect of a projected infrared watermarkusing a filter designed to block infrared light. Video cameras arenormally provided with some amount of IR filtering to compensate forsilicon sensitivity to IR.

Motion picture display and video recording standards have well-knownframe-to-frame refresh rates. In standard motion picture projection, forexample, each film frame is typically displayed for a time duration of1/24^(th) of a second. Respective refresh rates for interlaced NTSC andPAL video recording standards are 1/60^(th) of a second and 1/50^(th) ofa second. Video camera capabilities such as variable shutter speedsallow close synchronization of a video camera with film projection,making it easier for illegal copies to be filmed within a theater.Attempts to degrade the quality of such a copy include that disclosed inU.S. Pat. No. 5,680,454 (Mead) and U.S. Pat. No. 6,529,600 (Epstein),which disclose use of a pseudo-random variation in frame rate, causingsuccessive motion picture frames to be displayed at slightly differentrates than nominal. Using this method, for example, frame displayperiods would randomly change between 1/23^(rd) and 1/25^(th) of asecond for a nominal 1/24^(th) second display period. Timing shiftswithin this range would be imperceptible to the human viewer, butsignificantly degrade the quality of any copy filmed using a videocamera. The randomization proposed therein would preventresynchronization of a video camera to a changed display frequency.While these methods may degrade the image quality of a copy made byvideo camera, they also have limitations. As noted in the disclosure ofU.S. Pat. No. 5,680,454, the range of frame rate variability isconstrained, since the overall frame rate must track reasonably closelywith accompanying audio. Furthermore, a video camera can easily bemodified to so that its frame rate tracks with the motion picture.

U.S. Pat. No. 5,959,717 (Chaum) also discloses a method and apparatusfor copy prevention of a displayed motion picture work. The apparatus ofU.S. Pat. No. 5,959,717 includes a film projector along with a separatevideo projector. The video projector can be used, for example, todisplay an identifying or cautionary message or an obscuring patternthat is imperceptible to human viewers but can be recorded using a videocamera. Alternately, the video camera may even display part of themotion picture content itself. By controlling the timing of the videoprojector relative to film projector timing, a message or pattern can bemade that will be recorded when using a video camera, but will beimperceptible to a viewing audience. This method, however, requiresdistribution of a motion picture in multiple parts, which greatlycomplicates film replication and distribution. Separate projectors arealso required for the film-based and video-based image components,adding cost and complexity to the system and to its operation. Imagequality, particularly for large-screen environments, may not be optimalfor video projection, and alignment of both projectors to each other andto the display surface must be precisely maintained.

U.S. Published patent application No. 2002/0168069 describes anapparatus and method for displaying a copy-deterrent pattern within adigital motion picture in order to discourage recording of the motionpicture using a video camera or other sampling recording device. Thecopy-deterrent pattern comprises a plurality of pixels within each frameof the digital motion picture, and the displayed pixel intensities aremodulated at a temporal frequency using modulation characteristicsdeliberately selected to be imperceptible to human observers whilesimultaneously producing objectionable aliasing in any copy made using avideo camera. There are drawbacks in this approach. During recordingusing a video camera the camera shutter can be set to open long enoughto average out the modulation. For example, in the example shown in FIG.12 of the reference, the frame rate is 24 frames per second, and themodulation is at 96 Hz. If the camera shutter speed is set at 1/48 sec,then there will be no or minimal alias effect. Furthermore, if samplingrate is fast enough (e.g. 96 Hz) then all the information will berecorded in the pirate copy and the modulation in pixel intensities canbe removed by digital signal processing.

U.S. published application No. 2004/0033060 describes a method ofmodulation of a video signal with an impairment signal to increase thevideo signal masked threshold. With this arrangement, an impairmentsignal is applied to a version of the movie having a higher frame ratethan normal. The impairment signal is designed to produce an apparentmotion across the frame when applied at a high frame rate and less rapidapparent motion when viewed at a lower frame rate. When a person viewsthe high frame rate movie, the brain is capable of summing rapid framessuch that the impairment is not visible. When a lower frame rate videocamera records such a signal, it will not capture all of the images, butrather only particular ones and as such the summing of successive imagesto produce the proper result will not be achieved. However, with recentimprovements in CCD technology, the exposure period possible with videorecorders is much longer. Where previously a 60 frame per second camerawould have an exposure time that was much less than 1/60^(th) of asecond, new technologies allow exposures approaching the full 1/60^(th)of a second duration. In such a case, the video camera would be able toperform an integration to sum successive rapid frames such that theslower frames are substantially correct.

Conventional methods such as those described above could be adapted toprovide some measure of copy deterrence and watermarking for digitalmotion pictures. However, none of the methods noted above is whollysatisfactory, for the reasons stated.

SUMMARY OF THE INVENTION

According to one broad aspect, the invention provides a methodcomprising: for each of a plurality of still images of a motion picture:rendering a respective sequence of sub-frames such that each sub-framecomprises the still image with one or more portions omitted, with everyportion of the still image being included in at least one sub-frame;applying intensity compensation such that when the sequence ofsub-frames is displayed, an image substantially equivalent to the stillimage is perceptable.

In some embodiments, the method further comprises: processing each stillimage to determine portions that have a low intensity; wherein theportions that are omitted in rendering the sequence of sub-frames areselected from the portions determined to have low intensity.

In some embodiments, processing each still image to determine portionsthat have a low intensity comprises: determining portions that have anintensity less than or equal to 1/N times a maximum intensity, where Nis the number of sub-frames in the sequence of sub-frames.

In some embodiments, applying intensity compensation such that when thesequence of sub-frames is displayed, an image substantially equivalentto the still image is perceptable comprises: rendering a film with onestill image per processed still image in which portions that are to beomitted are brighter on the film; storing information identifying theportions of the film that are brighter; during projection, applyingsub-framing using the stored information.

In some embodiments, rendering and applying intensity compensationcomprises: storing one digital image per sub-frame; increasing intensityof stored pixels that are omitted in at least one sub-frame.

In some embodiments, applying intensity compensation and renderingcomprises: storing one digital image per still image in which pixels ofthe portions to be omitted have increased intensity, and storinginformation identifying the portions that have increased intensity;during projection, applying sub-framing using the stored information.

In some embodiments, processing the image to determine portions that areless bright comprises: determining a maximum brightness of each of a setof possible portions to be omitted; selecting portions having a maximumbrightness less than a threshold.

In some embodiments, determining a maximum brightness of each of a setof possible portions to be omitted comprises: determining a maximum Yvalue for each of the possible portions to be omitted.

In some embodiments, the method further comprises computing a Y valuefor each pixel from RGB coding.

In some embodiments, rendering the sub-frames comprises displaying thesub-frames.

In some embodiments, rendering comprises recording.

In some embodiments, the plurality of sub-frames of the still imagecomprises N sub-frames, wherein N is a positive integer greater than 1,and wherein each sub-frame is displayed for sub-intervals that isapproximately a fraction 1/Nth of an image time interval, and whereinportions that are omitted in all but one sub-frame have their intensityincreased by a factor of N^(1/γ), where γ is a power relationshipbetween an encoded luminance (Y) of the portion and an actual desiredimage brightness (I) byI=Y^(γ)

In some embodiments, the method further comprises generating each stillimage in a motion picture projector for projection onto a screen;wherein rendering comprises filtering the generated still image to allowthe plurality of portions of the generated still image to pass to thescreen during respective sub-intervals.

In some embodiments, the method further comprises: projecting each stillimage onto a mirror; wherein rendering comprises reflecting theplurality of portions of the still image from the mirror to a screenduring respective sub-intervals.

In some embodiments, rendering comprises illuminating a mirror in adigital motion picture projector.

In some embodiments, displaying comprises: providing an LCD (liquidcrystal display) covering at least a portion of the mirror; andcontrolling the liquid crystal display to position liquid crystals ofthe LCD to allow the portions of the generated still image to passthrough the LCD during respective sub-intervals.

In some embodiments, rendering comprises filtering each still image todisplay the plurality of portions of the still image during therespective sub-intervals.

Other aspects and features of embodiments of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments of the invention will now be described ingreater detail with reference to the accompanying diagrams, in which:

FIG. 1 is a representation of a set of sub-frames;

FIG. 2 shows the effect of using the set of sub-frames of FIG. 1;

FIG. 3 shows the effect of recording the pattern of FIG. 2 with a framesynchronization offset by one sub-frame;

FIG. 4 is an example of sub-frame sequencing provided by an embodimentof the invention;

FIG. 5 is a flow diagram illustrating a method according to anembodiment of the invention;

FIG. 6 is a block diagram of a system according to an embodiment of theinvention;

FIG. 7 is an embodiment of an apertured filter component; and

FIG. 8 is a block diagram of a system according to an embodiment of theinvention;

FIG. 9 is a block diagram of a digital projector in which embodiments ofthe invention may be implemented;

FIG. 10 is an illustration of how sub-frame intensity compensation canbe performed for an example image, as provided by an embodiment of theinvention;

FIG. 11 is a flowchart of a method of performing sub-frame intensitycompensation provided by an embodiment of the invention; and

FIGS. 12A and 12B are further flowcharts of another method of performingsub-frame intensity compensation as provided by another embodiment ofthe invention.

FIG. 13 is a flowchart of another method of performing sub-frameintensity compensation as provided by an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention generally relate to methods and systems fordisplaying portions of a still image instead of a complete still image,such as a set of sub-frames instead of a standard frame in a motionpicture. Each sub-frame does not have all the information of the frame,but the set of sub-frames preferably contains the same information asthe frame. To an audience viewing a motion picture displayed in thismanner, there is no visible difference, but for a video capture devicesuch as video camera, there will be information missing if not all thesub-frames are captured.

If a still image is divided into a collection of small colored dots,then the human brain is able to effectively reassemble the dots into ameaningful image. Similarly, if a moving scene is divided into asequence of still images which are shown in rapid succession, then thebrain reassembles the still images into a single moving scene.

Movies work because of persistence of vision. The human eye generallyretains an image for about 1/20^(th) of a second after seeing it. Suchpersistence of vision creates the illusion of motion from still images.This phenomenon, however, is not exhibited to the same extent inrecording devices such as a video camera. For example, when a videocamera records a moving scene from a TV or computer monitor, there tendsto be pronounced flicker and often a black rolling bar in the recordedimage instead of the stable image that eyes see.

This flicker is caused by two characteristics, namely a difference inthe scanning frequency between the TV or monitor and the camera, and adifference between the way in which phosphor dots are perceived by thehuman eye and the camera's image sensor. In a standard CRT display, asingle electron beam scans horizontal lines of pixels across the screen,lighting up each pixel when the beam hits it. The pixels are made ofindividual phosphor dots that glow when the beam hits them. In humaneyes, the dots glow for about 1/30^(th) of a second, so we see a steadyimage. For a video camera, however, the dots do not appear to glownearly as long. The camera is much less persistent than the eye.

With monitor refreshing every 1/60^(th) of a second and a camera takinga frame every 1/60^(th) of a second, for example, the video camera'slower persistence results in low quality recording. The black bar thatis observed when the recorded video is played back is essentially acollection of pixels that had faded by the time the camera tried toimage them. The bar rolls because the camera and monitor are not exactlysynchronized, even though the frame rates may be nominally the same.

In projecting a movie from film for instance, each frame is exposed fora short time and then advanced to the next frame. Normally, the film isadvanced at the rate of 24 frames per second.

If a regular repeating pattern of sub-frames is used, it is possiblethat a video capture device with a long enough exposure time will beable to capture all of the sub-frames. For example, referring to FIG. 1,suppose each original image frame 10 is sub-divided into five sub-frames12,14,16,18,20 according to a repeating pattern with each sub-framehaving portions of the original image blocked out as shown. The sum ofthe five images 12,14,16,18,20 adds up to the original frame 10.

Now referring to FIG. 2, when a sequence of sub-frames generallyindicated at 22 is displayed, with sets 24,26,28,30 of five sub-framesshown for each of four different frames, sub-frames 24 are perceived asthe original image 32, sub-frames 26 are perceived as image 34,sub-frames 28 are perceived as image 36 and sub-frames 30 are perceivedas original image 38. That is to say the fact that the sub-frames 22 areplayed at a rapid pace allows the brain to add consecutive sub-framessub-consciously to produce the original images. Thus, the fact that theimages have been transformed into the sub-frames with blocked outportions as shown in FIG. 1 does not result in a perceptable degradationin the picture quality.

Disadvantageously, with the sub-frame format of FIG. 1, if a videocapture device is successfully synchronized with the first sub-frame ofeach frame, and has an exposure time long enough to capture all fiveframes of each image, then no picture degradation will result in therecorded video. Furthermore, in the event the video camera is capable ofsynchronizing not to the first sub-frame of each original image, but toone of the other sub-frames, for example each second sub-frame, therestill would not be significant degradation. This is shown in FIG. 3where the exposure intervals are indicated at 50,52,54. Now it can beseen that during exposure interval 50, the second, third, fourth andfifth sub-frames of original image 24 and the first sub-frame of image26 are summed and recorded. During exposure interval 52, the second,third, fourth and fifth sub-frames of original image 26 and the firstsub-frame of image 28 are summed and recorded, and during exposureinterval 54, the second, third, fourth and fifth sub-frames of originalimage 28 and the first sub-frame of original image 30 are summed andrecorded. The resulting sum is indicated at 56,58,60 for the threeexposure periods. For each exposure period, four of the sub-frames areaccurately recorded, but the fifth sub-frame is from the followingimage. However, because consecutively images generally do not differ bythat much, the fact that a portion of the image is taken from afollowing image in a repeating manner will not significantly degrade theperception of the movie. In particular, it can be seen that theresulting recorded image for the first frame 24 contains three blocks68,70,72 (corresponding to the blocks of the first sub-frame 12 ofFIG. 1) for the succeeding frame 26 with a remainder of the image takenfrom the current image 24. So long as the two images are notsignificantly different, this will not create a significant perceptabledegradation in the image quality. Thus, the entire effort of degradingthe image does not achieve the desired result of deterring video piracy.

Referring now to FIG. 4, in an effort to deal with this, an element ofvariation is introduced into the sub-frame sequencing. This will bedescribed with the specific example of FIG. 4 with more general conceptsintroduced later. In this case, there are again four sets of fivesub-frames 80,82,84,86. Each of these sub-frames contains a respectiveportion of an original image. For the purpose of this example, it isassumed that the sub-frames are produced using the sub-frame patternsshown in FIG. 1. However, in this case, the sequence of sub-frames foreach frame is not repeating. Rather, there is a variation in the orderof the sub-frames between adjacent images. For the first image 80, thesequence of sub-frames is 1,2,3,4,5. For the next image 82, the sequenceis 2,5,3,1,4. For the next image, 84, the sequence is 5,1,4,3,2.Finally, for the next image 86, the sequence is 1,4,5,2,3. The effect ofthis upon a video recording of such a sequence of sub-frames can be seenat 90. In this case, it is assumed that the video recording device iscapable of synchronizing on the second sub-frame of each frame. Thismeans that during the first exposure interval 92, sub-frames 2,3,4,5 ofthe first image 80 and sub-frame 2 of the second image 82 are summed toproduce image 94. It can be seen that such a sum has gaps in the imageindicated at 96,98,100 due to the fact that the sum does not includesub-frame 1 from either the current image 80 or from a following image82. Furthermore, there is also degradation in the areas indicated at102,104,106 where the contribution of the second sub-frame of each oftwo successive frames 80,82 has been summed resulting in distortion. Itcan be seen that the randomness introduced in the sub-frame orderresults in the degradation in each frame with the degradation beingdifferent in successive frames.

In some embodiments, the fast response of advanced LCDs (liquid crystaldisplays) and DLPs (digital light processors) is exploited to display anumber of sub-frames instead of a standard frame in a motion picturewithin a standard frame period of 1/24^(th) of a second, which may bereferred to more generally as an image time interval.

FIG. 5 is a flow diagram illustrating a method according to anembodiment of the invention. At 10, one of a number of still images of amotion picture is processed for display during an image time interval,illustratively the frame duration described above to produce sub-frameswith variation in the manner in which still images are divided intosub-frames. Examples of this variation include, but are not limited to:a) variation in the number of sub-frames per image interval; b)variation in the impairment pattern applied to successive sub-frames(specific example given in FIG. 4); c) variation in the duration of thesub-frames per image interval. Portions of the generated still image arethen displayed at 12 during respective sub-intervals of the image timeinterval. Thus, in each sub-interval, only a portion of the generatedstill image is displayed. As indicated at 14, these operations arerepeated for each still image in the motion picture.

Generally, an image is separated into n portions, and each of thesub-intervals is preferably approximately a fraction 1/nth of the imagetime interval, where n is a positive integer greater than one. However,it should be appreciated that the portions need not be of equal size,and that the sub-intervals may also be of different lengths.

In another embodiment, the sub-frames of a given image interval onaverage are displayed during a period equal to the image interval, butwith a small variation in the period used for successive images. Thistype of variation will not be perceptable to a user, but will furtherdegrade the ability of a video capture device to synchronize to a frameor sub-frame rate.

In one embodiment, a sequence of impairment masks is applied to eachimage to produce a set of sub-frames for each interval with the order ofthe resulting sub-frames being randomized between successive imageintervals.

The order does not need to be truly random. Rather, there needs to besome variation between at least some, and preferably all, consecutiveimage intervals to achieve the desired effect.

In one embodiment, the still image is generated in a projector forprojection onto a screen. The operation at 12 in such embodiments mayinclude filtering the generated still image to allow the portions of thegenerated still image to pass to the screen one at a time during thesub-intervals. This filtering may be accomplished, for example, byproviding an apertured filter component and moving the apertured filtercomponent to position apertures such that only a portion of thegenerated still image is allowed to pass to the screen during each ofthe sub-intervals. Such a filter component may be provided either withinthe projector or as an external component, such as in an accessory forthe projector, and is preferably but not necessarily synchronized withthe projector, such as once per image time interval. For a filmprojector, for example, movement of a filter component is preferablysynchronized with the frame rate. Illustrative examples of a filtercomponent are described in further detail below.

According to other embodiments of the invention, a projector projectsgenerated still images onto a mirror, which may be internal or externalto the projector. Portions of the still images are then reflected fromthe mirror to a screen during respective sub-intervals of an image timeinterval. Control of the portions that are reflected to the screenduring any sub-interval may be provided by an LCD covering at least aportion of the mirror. Liquid crystals of the LCD are then positioned toallow the portions of the still image to pass through the LCD during therespective sub-intervals. In another embodiment, the mirror includesmultiple mirrors with controllable angles, in a DMD (digital micromirrordevice) for instance. Each mirror is positioned at a particular angleduring each sub-interval to reflect a portion of the generated stillimage to the screen.

Each image time interval preferably has substantially the same duration.Similarly, each sub-interval of an image time interval also preferablyhas substantially the same duration, which is less than the image timeinterval. However, it should be appreciated that the invention is in noway limited thereto. Embodiments of the invention may be used inconjunction with variable frame rate projectors, with synchronization atthe beginning of each image time interval or frame for example, and maydisplay portions of an image for different amounts of time.

FIG. 6 is a block diagram of a system according to an embodiment of theinvention. The system includes an image generator 16 operatively coupledto a display controller 18 via a connection 17. Of course, a motionpicture display or projection system may include further elements thanthose explicitly shown in FIG. 6.

The image generator 16 generates still images of a motion picture fordisplay during respective image time intervals. In a preferredembodiment, the image generator 16 is a motion picture projector such asa film projector which illuminates motion picture film or a digitalprojector. One example of a digital projector is described in furtherdetail below with reference to FIG. 7.

Although shown and referred to as a connection, the connection 17 shouldbe understood to include a physical connection, an optical connectionwith or without an actual physical carrier connected between the imagegenerator 16 and the display controller 18, or possibly combinationsthereof or a logical connection. For example, a physical connection maybe provided to enable synchronization between the image generator 16 andthe display controller 18, whereas still images generated by the imagegenerator 16 are provided to the display controller 18 through anoptical “line-of-sight” path.

The display controller 18 displays portions of each still imagegenerated by the image generator 16 during respective sub-intervals ofthe image time interval in which the still image is generated. Thus, thedisplay controller 18 effectively separates generated still images intoportions for display in corresponding sub-intervals. The displaycontroller 18 may be implemented in a projector which also incorporatesthe image generator 16, or as an external accessory for such aprojector.

In one embodiment, the display controller 18 includes a filter forfiltering each generated still image. Portions of each generated stillimage are thus passed, to a projection screen for example, duringrespective sub-intervals. Such a filter may include an apertured filtercomponent, an illustrative example of which is shown in FIG. 7.Apertured filter component 30 includes apertures 32,34,36 that defineone portion or sub-frame. The portion consists of three square areas32,34,36. Further filters would be defined for the other sub-frames. Thefilters are applied in sequence to the still image. The filters mightfor example be implemented with a transparent liquid crystal display.Alternatively, motion between a series of different physically distinctfilters may be used. Those skilled in the art will appreciate thatdifferent numbers, shapes, and sizes of apertures and filter componentsthan those specifically shown may be used for display control inaccordance with the techniques disclosed herein. The size of theaperture or apertures determines the number of portions into which animage is separated.

Other forms of filter components will be apparent to those skilled inthe art to which this application pertains. It should thus beappreciated that the invention is in no way limited to the filtercomponent 30.

Where physically distinct components are provided, the displaycontroller 18 moves the filter components during each image timeinterval to position any apertures to pass or transmit portions of agenerated still image during respective sub-intervals. As will beapparent, the filter components may be moved through control of a motor,linkage, or other device (not shown) for moving the filter component.

According to further embodiments of the invention, the image generator16 projects an image onto a mirror, which reflects the image onto ascreen. In one embodiment, an LCD covers at least a portion of themirror, and the display controller 18 controls the LCD to positionliquid crystals of the LCD so as to allow portions of the image to passthrough the LCD to the screen during respective sub-intervals an imagetime interval. It should also be appreciated that an LCD may be used ina similar manner on its own as a filter component as discussed above.

The mirror includes multiple mirrors in another embodiment of theinvention, and the display controller 18 controls an angle of eachmirror to reflect portions of the image to the screen during therespective sub-intervals.

FIG. 8 is a block diagram of a system incorporating an embodiment of theinvention. In the system of FIG. 8, a DMD or mirror/LCD combination 42is provided externally to a projector 40, and reflects portions ofimages generated by the projector 40 onto the screen 44. This type ofsystem is particularly adapted to projectors which do not incorporate amirror, such as film projectors, for example.

As will be apparent from FIG. 8, an image separator, which includes amirror in the example system of FIG. 8 but may instead include a filtercomponent or other type of separator, may be provided externally to aprojector. However, digital projectors incorporate such mirrors. FIG. 9is a block diagram of a digital projector in which embodiments of theinvention may be implemented.

There are currently two major digital cinema projector technologies:micromirror projectors and LCD projectors. Micromirror projectors formimages with an array of microscopic mirrors. In this type of system asshown in FIG. 9, a high-power lamp 100 shines light through a prism 102.The prism 102 splits the light into the component colors red, green andblue. Each color beam hits a different DMD 104, which is a semiconductorchip that is covered in hinged mirrors, often more than a million hingedmirrors. Based on information encoded in a video signal, the DMD turnsover the tiny mirrors to reflect the colored light. Collectively, thetiny dots of reflected light form a monochromatic image.

In actuality, most of the individual mirrors are flipped from “on”(reflecting light) to “off” (not reflecting light, or reflecting lightto a location outside an area of a projection screen) and back againthousands of times per second. A mirror that is flipped on a greaterproportion of the time will reflect more light and so will form abrighter pixel than a mirror that is not flipped on for as long. This ishow the DMD creates a gradation between light and dark. The mirrors thatare flipping rapidly from on to off create varying shades of gray (orvarying shades of red, green and blue, in this case). Each micromirrorchip reflects the monochromatic image back to the prism, whichrecombines the colors. The red, green and blue rejoin to form a fullcolor image, which is projected onto a screen.

LCD projectors reflect high-intensity light off of a stationary mirrorcovered with an LCD. Based on a digital video signal, the projectordirects some of the liquid crystals to let reflected light through andothers to block it. In this way, the LCD modifies the high-intensitylight beam to create an image.

Since signal information is already encoded in digital format fordigital projectors, and each micromirror (in micromirror technology) orpixel (in LCD technology) is independently addressable, embodiments ofthe invention can be easily incorporated into digital projectors.

According to a further embodiment of the invention, portions of a stillimage or frame of a motion picture, the still image or frame having anassociated image or frame rate, are generated. The portions of the stillimage or frame are then displayed during respective sub-image or frametime intervals at a sub-image or sub-frame rate which is greater thanthe image or frame rate. Where there are n portions (n a positiveinteger greater than one), the sub-image or sub-frame rate is preferablyn times the image or frame rate.

Such an embodiment of the invention may be implemented, for example, byrecording the portions of the image or frame on a recording medium andthen operating a projector at the sub-image or sub-frame rate instead ofthe image or frame rate. The recording medium may be motion picture filmor a digital recording medium such as a DVD (digital video disk) forinstance.

In another embodiment, the manner in which the sub-frames is generatedcan be changed in real time locally and/or remotely.

For example, although described primarily in the context of separatingan entire image into separately displayed portions, the techniquesdescribed herein may alternatively be applied to only a particularsection of an image, with other sections of the image being displayedduring an entire image time interval. In this case, illuminationstrengths are preferably controlled to avoid any perceptible differencesin intensity between the sections of the image.

In the embodiments described, the sub-frames are generated by firstgenerating the still image and then applying a sequence of impairmentsto generate these sub-frames. More generally, any method of renderingthe sub-frames may be employed. For example, the film may be produced tohave a higher frame rate in which these sub-frames are present. It maybe that the entire image is never actually generated, but rather thesequence of sub-frames alone is generated. The rendered images may insome embodiments be projected or otherwise displayed. Alternatively, therendered images may be recorded for later display. This recording cantake place in any suitable recording medium.

Preferably, the variation in the manner in which the portions of thestill image are generated is selected such that a gap left in a firststill image due to the omission of a first sub-frame of the sequence ofsub-frames for that still image is not filled by a first sub-frame ofthe next still image. Preferably, this variation is present for multiplepairs of consecutive images and in some embodiments for all pairs ofconsecutive images.

In some embodiments, the portion shapes used for rendering thesub-frames of each still image are the same. In such an embodiment, thevariation in the manner in which the respective portions are generatedis achieved by rendering the sequences of sub-frames to have a variationin the order of the set of portion shapes among the pairs of consecutivestill images. For example, if five different portion shapes are usedsuch as was the case in the example of FIG. 4, the desired effect can beachieved by employing a different order among the portion shapes for thesub-frames of consecutive images.

In another embodiment, there is no relationship between the sub-frames,this not being the case for the “picket fence” approach taught inabove-referenced application No. 2004/0033060.

In another embodiment, there is randomness or pseudo-randomness in thesequences of sub-frames.

In addition, enhanced control of image separation and illuminationintensity may be provided to implement variable separation schemes,wherein a number of image portions and sub-intervals are varied duringprojection of a motion picture.

Sub-Frame Intensity Compensation

In some of the above described embodiments, each frame is divided into anumber of sub-frames (N). Each sub-frame contains only part of thepicture and is exposed only 1/Nth of the frame duration. Otherembodiments do not necessarily require equal sub-frame exposure. The sumof all sub-frames equals the whole original frame. However, theeffective exposure time for the whole frame is only 1/Nth that of theoriginal frame. In some embodiments, to compensate for this an increasedlight intensity is employed, for example by using a more powerful lamp.In another embodiment, the intensity of portions of the original pictureis modified to compensate for the reduction of exposure time. This ispossible because the mind effectively performs an integration over theframe length such that something displayed more brightly for a shorterperiod of time will be perceived the same as something displayed lessbrightly for a shorter period of time.

Frames collectively will have regions that are relatively bright, andregions that are relatively less bright. For areas that are alreadybright, it may not be possible to have their intensity adjusted tocompensate for the reduction of exposure time since there is a maximumbrightness. However, for areas that are less bright, they can have theirintensity adjusted to compensate for the reduction in exposure time,since there is “room” to increase the brightness before reaching themaximum brightness. For frames that do not have any dark areas,preferably no sub-framing is applied. Equivalently, a set of identicalsub-frames with no omitted portions can be displayed. This is not verycommon in a typical movie.

FIG. 11 shows a flowchart of a method of compensating for the reductionof exposure time as provided by an embodiment of the invention. In step11-1, the next image to be processed is obtained. At step 11-2, theimage is processed to identify areas that have low intensity, low enoughthat they can be intensity compensated. This may for example involveidentifying portions having a maximum brightness less than somethreshold. In some embodiments, assuming that the picture is to bedivided into N sub-frames, step 11-2 can be implemented by identifyingthe regions whose brightness is less than 1/Nth of a maximum brightness.Preferably this is done to a resolution equal to that to be used forsub-frame construction described below. For example, if sub-frameconstruction involves dividing each frame into a 3×3 array of portions,and selecting a sub-set of these portions for each sub-frame, thenpreferably, the image processing in step 11-2 is performed on this same3×3 array of portions to obtain a brightness for each portion and toidentify which portions have low intensity. At step 11-3, a sub-framepattern is then defined in such a manner that it is the portions withlow intensity that are selectively removed from some of the sub-frames.At the same time, the brighter portions are included in all thesub-frames produced for that image. Every portion is included in atleast one sub-frame. At step 11-4, the intensity compensation is appliedsuch that when the sequence of sub-frames is displayed, an imagesubstantially equivalent to the still image is perceived. For example,this may involve increasing the brightness of each portion that has beenremoved from one or more sub-frames in at least one of the sub-frame(s)in which it remains to compensate for the portion having being omittedin one or more other sub-frames.

Note that in implementations where maximum brightness is not a factor,the above method can be implemented without consideration to thebrightness of the portions being omitted. In other words, portions canbe arbitrarily omitted, with appropriate intensity compensation to dealwith the shorter exposure time.

An example of this more general application is shown in the flowchart ofFIG. 13. The method involves first getting the next image at step 13-1.At step 13-2, a sequence of sub-frames as rendered such that eachsub-frame comprises the still image with one or more portions omittedwith every portion of the still image being included in at least onesub-frame. Note that in this case, the portions being omitted may or maynot be selected as a function of intensity of the original image. Atstep 13-3, intensity compensation is then applied such that when thesequence of sub-frames is displayed, an image substantially equivalentto the still image is perceptable. Note that the flowchart shows aparticular order of steps, but that the order of steps 13-2 and 13-3 canbe reversed, and/or these steps can be combined. For example, theintensity compensation can be applied to areas of the still image thatare to be omitted in one or more frames prior to the sub-framingoperation.

In some applications image coding is based on the YUV model. Each spot(pixel) is represented by three numbers (Y,U,V). Y stands for theluminance component (the brightness) and U and V are the chrominance(color) components. In one example of how the maximum brightness can bedetermined for such applications, the maximum brightness within an areais just the maximum value of Y in that area.

More generally, any appropriate method of determining brightness can beemployed.

YUV signals are created from an original RGB (red, green and blue)source. The weighted values of R, G and B are added together to producea single Y signal, representing the overall brightness, or luminance, ofthat spot. By examining the value of Y, the brightness of that spot canbe determined.

In another example, if the coding is based on the RGB model, therelationship between YUV and RGB can be calculated by the followingequations and the brightness can be easily determined based on the Yvalue calculated. Of course, if only Y is needed then U and V need notbe determined. $\begin{matrix}{Y = {{0.299R} + {0.587G} + {0.114B}}} \\{U = {0.492\left( {B - Y} \right)}} \\{= {{{- 0.147}R} - {0.289G} + {0.436B}}} \\{V = {0.877\left( {R - Y} \right)}} \\{= {{0.615R} - {0.515G} - {0.100B}}}\end{matrix}$

An example of an application of a very particular instantiation of themethod will now be illustrated by way of example with reference to FIG.10. Generally indicated at 200 is an example of a still image havingregions of different brightness for example ranging from 1 to 9 asshown. It is assumed that “9” is also the maximum brightness that can beaccommodated. For this example, it is assumed that sub-frame portionsare defined in a 3×3 array of portions labelled P1, P2, . . . , P9. Thebrightness of each sub-frame portion is the maximum brightness withinthe portion and is indicated in the 3×3 array 201, with the left mostcolumn of portions P1, P4, P7 having a maximum brightness of “3”, themiddle column of portions P2, P5, P8 having a maximum brightness of “6”,and the right most column of portions P3, P6, P9 having a maximumbrightness of “9”. In this case, the three portions P1, P4, P7 having amaximum brightness of “3” are available for sub-frame intensitycompensation, since they have a maximum brightness less than or equal to1/N'maximum brightness=⅓×9=3. A sub-framing pattern is then defined thatproduces sub-frames that omit one or more of these three portions P1,P4, P7. A three-sub-frame pattern is indicated at 204,206,208 in whichsub-frame 204 includes P1, but omits P4 and P7, sub-frame 206 includesP4, but omits P1 and P7, and sub-frame 208 includes P7 but omits P1 andP4. In the sub-frames 204,206,208, any portion that is omitted at leastonce has its brightness increased to compensate for the decreasedexposure time. This is shown having been done in the sub-frames204,206,208 where portions P1, P4, P7 are shown with three times theiroriginal brightness. Equivalently, the original picture can be adjustedprior to sub-framing as shown at 202 where portions P1, P4, P7 are shownwith increased brightness.

When the sub-frames 204,206,208 are displayed in sequence, assumingproper selection of the intensity adjustment and exposure times of thesub-frames 204,206,208 the perceived effect will be the same as if theoriginal picture 200 had been exposed for the entire time.

The relationship between the encoded luminance of the picture and theactual desired image brightness is a power-law relationship i.e.I=Y^(γ)where I is the image brightness, Y is the encoded luminance of thepicture and γ is the power relationship, with a typical value of γ beingabout 2.5. γ depends on how the brightness is being coded. To increasethe brightness by a factor of N, Y has to be increased by N^(1/γ). Forexample, if N=3, and γ is 2.5 then Y is increased by 1.5518.Digital Implementations

For digital projectors, the method can be applied in real time, or theprocessing can be performed off-line. In real-time implementations, theframes are analyzed and the regions of low intensity are mapped in realtime, and sub-framing is applied to those regions.

Film Implementations

The approach can also be incorporated in implementations employing filmprojectors. In film production, typically at some point the video isconverted into digital format where special effects are added and videosare edited. The finished video is then converted into film. At thedigital stage, the frame can be analyzed and the low intensity region ismapped and its intensity is increased appropriately. The information isstored and used to control the sub-frame generation during projection.

Referring to FIGS. 12A and 12B, flowcharts of method suitable for filmimplementations are shown, although these methods would also beapplicable to digital projectors. The method of FIG. 12A is performed toprepare the film, and the method of FIG. 12B is performed whenprojecting the film.

Referring first to FIG. 12A, at step 12A-1, the next image is obtained;it is assumed that the images are still available in digital form. Atstep 12A-2, the image is processed to identify low intensity portions.At step 12A-3, the brightness of the low intensity portions is increasedat some point prior to rendering the film version of the image. At step12A-4, information is stored that identifies the portions of the imagethat were brightened. For digital projector implementations, this can bestored in the digital video file whereas for analog projectorimplementations a separate file is required. For the particular exampleof FIG. 10, the information might be that for image number <imagenumber>, portions P1,P4 and P7 had their brightness increased by afactor of 3.

Referring next to FIG. 12B, at step 12B-1 the next image is obtained.For film projector implementations, this involves the next image beingprojected by the film projector. While the image is being projected,sub-framing is performed using the previously stored information withthe result that for the portions that had their brightness increased,they are exposed for proportionally less time such that an effectequivalent to that of the overall original image is achieved. Any of thedisplay methods described previously can be employed.

In some embodiments, the portions are defined to align with segmentationperformed by a compression algorithm. For example, MPEG appliescompression on a block-wise basis, and preferably the sub-framing isperformed using these same blocks.

Another embodiment of the invention provides a projection system adaptedto implement any one of the methods described herein, such as the methodof FIGS. 11, 12 or 13. For such embodiments, the sub-framing is appliedin real time.

Another embodiment of the invention provides a motion picture mediumproduction system adapted to implement any one of the methods describedherein. For such embodiments, sub-framing and/or intensity compensationis performed while producing the motion picture medium that isultimately distributed for projection.

The system of FIG. 6 can also be adapted to implement the intensitycompensation methods. The image generator generates a plurality of stillimages of a motion picture for display during respective image timeintervals. The display controller displays, for each of a plurality ofstill images of a motion picture, a respective sequence of sub-framessuch that each sub-frame comprises a respective portion of the stillimage and such that all portions of the still image are included in atleast one sub-frame. Intensity compensation is applied such that eachportion of the still image that is not included in all sub-frames isperceptable to have a brightness substantially equivalent to what wouldhave been the case had the portion been included in all sub-frames. Thiscan be done a priori, or during projection.

Further embodiments of the invention provide various motion picturemedia. Such media include structural features that render them toachieve certain functionality. These can be implemented as film, or asdigital media storing digital images.

In one example, a motion picture medium is provided that consists of,for each of a plurality of still images, a respective sequence ofsub-frames such that each sub-frame comprises a respective portion ofthe still image and such that all portions of the still image areincluded in at least one sub-frame. Intensity compensation is applied tothe sub-frames such that each portion of the still image that is notincluded in all sub-frames is perceptable to have a brightnesssubstantially equivalent to what would have been the case had theportion been included in all sub-frames.

In another example, an intensity compensated sub-framed motion picturemedium that has a plurality of still images in which intensitycompensation has been performed such that when subsequently a respectivesequence of sub-frames is produced and displayed from each still imagesuch that each sub-frame comprises a respective portion of the stillimage and such that all portions of the still image are included in atleast one sub-frame, each portion of the still image that is notincluded in all sub-frames is perceptable to have a brightnesssubstantially equivalent to what would have been the case had theportion been included in all sub-frames. There is a computer readablemedium containing information on how sub-framing is to be performedconsistently with how intensity compensation was performed.

In yet another example, an intensity compensated sub-framed motionpicture medium is provided that consists of for each of a plurality ofstill images, a respective sequence of sub-frames such that eachsub-frame comprises a respective portion of the still image and suchthat all portions of the still image are included in at least onesub-frame. There is also a computer readable medium containinginformation identifying where intensity compensation needs to beperformed within the sub-frames such that each portion of the stillimage that is not included in all sub-frames is perceptable to have abrightness substantially equivalent to what would have been the case hadthe portion been included in all sub-frames.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A method comprising: for each of a plurality of still images of amotion picture: rendering a respective sequence of sub-frames such thateach sub-frame comprises the still image with one or more portionsomitted, with every portion of the still image being included in atleast one sub-frame; applying intensity compensation such that when thesequence of sub-frames is displayed, an image substantially equivalentto the still image is perceptable.
 2. The method of claim 1 furthercomprising: processing each still image to determine portions that havea low intensity; wherein the portions that are omitted in rendering thesequence of sub-frames are selected from the portions determined to havelow intensity.
 3. The method of claim 2 wherein processing each stillimage to determine portions that have a low intensity comprises:determining portions that have an intensity less than or equal to 1/Ntimes a maximum intensity, where N is the number of sub-frames in thesequence of sub-frames.
 4. The method of claim 1 wherein applyingintensity compensation such that when the sequence of sub-frames isdisplayed, an image substantially equivalent to the still image isperceptable comprises: rendering a film with one still image perprocessed still image in which portions that are to be omitted arebrighter on the film; storing information identifying the portions ofthe film that are brighter; during projection, applying sub-framingusing the stored information.
 5. The method of claim 1 wherein renderingand applying intensity compensation comprises: storing one digital imageper sub-frame; increasing intensity of stored pixels that are omitted inat least one sub-frame.
 6. The method of claim 1 wherein applyingintensity compensation and rendering comprises: storing one digitalimage per still image in which pixels of the portions to be omitted haveincreased intensity, and storing information identifying the portionsthat have increased intensity; during projection, applying sub-framingusing the stored information.
 7. The method of claim 1 whereinprocessing the image to determine portions that are less brightcomprises: determining a maximum brightness of each of a set of possibleportions to be omitted; selecting portions having a maximum brightnessless than a threshold.
 8. The method of claim 7 wherein determining amaximum brightness of each of a set of possible portions to be omittedcomprises: determining a maximum Y value for each of the possibleportions to be omitted.
 9. The method of claim 8 further comprisingcomputing a Y value for each pixel from RGB coding.
 10. The method ofclaim 1 wherein: rendering the sub-frames comprises displaying thesub-frames.
 11. The method of claim 1 wherein rendering comprisesrecording.
 12. The method of claim 5, wherein the plurality ofsub-frames of the still image comprises N sub-frames, wherein N is apositive integer greater than 1, and wherein each sub-frame is displayedfor sub-intervals that is approximately a fraction 1/Nth of an imagetime interval, and wherein portions that are omitted in all but onesub-frame have their intensity increased by a factor of N^(1/γ), where γis a power relationship between an encoded luminance (Y) of the portionand an actual desired image brightness (I) byI=Y^(γ)
 13. The method of claim 1, further comprising generating eachstill image in a motion picture projector for projection onto a screen;wherein rendering comprises filtering the generated still image to allowthe plurality of portions of the generated still image to pass to thescreen during respective sub-intervals.
 14. The method of claim 1,further comprising: projecting each still image onto a mirror; whereinrendering comprises reflecting the plurality of portions of the stillimage from the mirror to a screen during respective sub-intervals. 15.The method of claim 1, wherein rendering comprises illuminating a mirrorin a digital motion picture projector.
 16. The method of claim 15,wherein displaying comprises: providing an LCD (liquid crystal display)covering at least a portion of the mirror; and controlling the liquidcrystal display to position liquid crystals of the LCD to allow theportions of the generated still image to pass through the LCD duringrespective sub-intervals.
 17. The method of claim 1, wherein renderingcomprises filtering each still image to display the plurality ofportions of the still image during the respective sub-intervals.
 18. Aprojection system adapted to implement the method of claim
 1. 19. Amotion picture medium production system adapted to implement the methodof claim
 1. 20. A system comprising: an image generator configured togenerate a plurality of still images of a motion picture for displayduring respective image time intervals; and a display controllerconfigured to display, for each of a plurality of still images of amotion picture, a respective sequence of sub-frames such that eachsub-frame comprises a respective portion of the still image and suchthat all portions of the still image are included in at least onesub-frame; wherein there is intensity compensation such that eachportion of the still image that is not included in all sub-frames isperceptable to have a brightness substantially equivalent to what wouldhave been the case had the portion been included in all sub-frames. 21.The system of claim 20, wherein the plurality of sub-frames of eachgenerated still image comprises n sub-frames, and wherein each of thesub-frames is displayed for approximately a fraction 1/nth of the imagetime interval.
 22. The system of claim 20, wherein the image generatorcomprises a motion picture projector.
 23. An intensity compensatedsub-framed motion picture medium comprising: for each of a plurality ofstill images, a respective sequence of sub-frames such that eachsub-frame comprises a respective portion of the still image and suchthat all portions of the still image are included in at least onesub-frame; wherein intensity compensation is applied to the sub-framessuch that each portion of the still image that is not included in allsub-frames is perceptable to have a brightness substantially equivalentto what would have been the case had the portion been included in allsub-frames.
 24. An intensity compensated sub-framed motion picturemedium comprising: a plurality of still images in which intensitycompensation has been performed such that when subsequently a respectivesequence of sub-frames is produced and displayed from each still imagesuch that each sub-frame comprises a respective portion of the stillimage and such that all portions of the still image are included in atleast one sub-frame, each portion of the still image that is notincluded in all sub-frames is perceptable to have a brightnesssubstantially equivalent to what would have been the case had theportion been included in all sub-frames; a computer readable mediumcontaining information on how sub-framing is to be performedconsistently with how intensity compensation was performed.
 25. Themotion picture medium of claim 24 wherein the plurality of still imagescomprise a film.
 26. The motion picture medium of claim 24 wherein theplurality of still images comprise digital images stored together withthe information on how sub-framing is to be performed.
 27. An intensitycompensated sub-framed motion picture medium comprising: for each of aplurality of still images, a respective sequence of sub-frames such thateach sub-frame comprises a respective portion of the still image andsuch that all portions of the still image are included in at least onesub-frame; a computer readable medium containing information identifyingwhere intensity compensation needs to be performed within the sub-framessuch that each portion of the still image that is not included in allsub-frames is perceptable to have a brightness substantially equivalentto what would have been the case had the portion been included in allsub-frames.
 28. The motion picture medium of claim 27 wherein the stillimages are digital images stored together with the information.